The use of pyrethroids (synthetic pyrethrins) for the control of agricultural insect pests on crops has dramatically increased in recent years. As a class of compounds, they are by far the most rapidly growing group of insecticides and are expected to capture within a few years as much as half of the total insecticide market. Their advantages are a high degree of insecticidal activity and, generally, low mammalian toxicity when compared to other insecticides, e.g., organophosphates. A serious disadvantage is their high fish toxicity which severely limits their use on crops grown in or near water, as for example rice (control of rice water weevil).
Microencapsulation as a technique for formulating pesticides has been applied to organophosphate insecticides, such as diazinon and methyl parathion, and the resultant products are commericalized for agricultural and structural pest-control uses as KNOX OUT.RTM. 2FM insecticide and PENNCAP.RTM. M insecticide (Registered Trademarks of Pennwalt Corporation), respectively. This technique has been shown to impart lengthened biological effectiveness to these pesticides and to reduce their mammalian toxicity. Thus, it has been found that KNOX OUT.RTM. 2FM insecticide is more than 16 times less toxic orally and more than 8 times less toxic dermally than conventional formulations of the diazinon active ingredient. Similarly, PENNCAP.RTM. M insecticide is at least 5 times less toxic orally and 10 times less toxic dermally than conventional formulations of the unencapsulated methyl parathion active ingredient. Thus, microencapsulation can be expected to generally reduce mammalian toxicity by a factor of 5 to ca. 20 times. On the other hand, fish toxicity of methyl parathion as measured by 96-hour exposure to trout in standard tests is virtually unaffected by microencapsulation. Thus, in this test the concentrations of active ingredient required to kill 50% of the fish are 6.44 ppm and 5.16-8.19 ppm for the encapsulated and conventional methyl parathion formulations, respectively.
Quite unexpectedly, it has been discovered that the microencapsulation of pyrethroids results in a reduction of fish toxicity that far exceeds the above-mentioned values. Thus, in a 96-hour exposure of trout to water containing an unencapsulated, emulsifiable-concentrate of pyrethroid (permethrin), 0.015 mg of the active ingredient per liter of water resulted in the death of 50% of the fish present. In contrast, none of the trout exposed to the encapsulated permethrin formulation died, even at the highest concentration, 18.5 mg of active ingredient per liter. The difference in acute toxicity is, thus, in excess of 1,200-fold. In another test using a different formulation of microencapsulated permethrin, the difference between concentrations required to cause 50% of the fish to die was 4,800-fold. This dramatic reduction in fish toxicity, therefore, exceeds by far the previously observed decrease in toxicity that is typical for microencapsulated organophosphates.
While no satisfactory explanation has been found for this unexpectedly low fish toxicity, these microencapsulated pyrethroids, such as permethrin, have excellent biological activity against target insect species while maintaining a low level of mammalian toxicity.
This unexpected discovery has important economic consequences in that these increasingly important insecticides are now useful in applications for which they are currently not approved for use due to high fish toxicity.